I think part of it is individuals with mainstream browser settings thinking that because Chrome and reddit remember them every time due to the lack of privacy oriented settings on their system that that happens to everyone. They might have to hit login once every few months and forget what it's like.

Others get so caught up in their own usage habit that they forget others don't want to be prompted to login when they hit reply to a comment (which causes a page reload and they lose their place in the comments) but would rather login up front before commenting.